Tilt detection system, tilt detection method and tilt adjustment method

ABSTRACT

A tilt detection system includes an optical disk having a series of data recorded therein, the series of data having a pulse width smaller than the minimum pulse width of data recorded in an ordinary optical disk; a reproducing part reproducing a signal from the optical disk by emitting a light beam onto the optical disk; and a tilt detecting part detecting a tilt of the light beam with respect to the optical disk based on a reproduced RF signal provided by the reproducing part when the series of data having the smaller pulse width is reproduced.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to tilt detection system, tilt detectionmethod and tilt adjustment method in an optical disk drive fordetecting/adjusting a tilt of a light beam with respect to a diskrecording surface.

2. Description of the Related Art

A light beam emitted from a semiconductor laser is incident on arecording surface of an optical disk at right angles through anobjective lens for reading a signal recorded on the optical disk.However, when the light beam is not incident on the recording surface ofthe optical disk at right angles due to inclination of the optical diskand/or inclination of the objective lens, coma aberration is generated.As the coma aberration increases, the cross talk increases, and the S/Nratio of the read signal is degraded. Thereby, distortion occurs in thereproduced waveform, and jitter occurs, as is well known.

Therefore, in order to adjust the tilt of light beam with respect to theoptical disk, ordinarily, the jitter amount of the RF waves reproducedfrom the optical disk is measured for radial direction and tangentialdirection of the optical disk (hereinafter, the term ‘of the opticaldisk’ in ‘radial/tangential direction of the optical disk’ beingomitted) separately, with the optical disk in the reproduced condition,and the respective tilts are adjusted so that the jitter amount becomesminimum. This adjustment may be made as a result of the pickup of theoptical disk drive being inclined or as a result of the spindle motor ofthe optical disk drive being inclined.

FIGS. 1A and 1B show the jitter amount of the reproduced RF signal withrespect to the tilt in radial direction and the jitter amount of the RFsignal with respect to the tilt in tangential direction, respectively,at the time of tilt adjustment. The jitter amount is minimum when thetilt is 0, and the jitter amount increases as the tilt increases. Theinclination of the pickup or spindle motor for radial direction andtangential direction is adjusted, where the state (inclination) of thepickup or spindle motor in which the jitter amount is minimum isregarded as the optimum state in tilt adjustment.

As shown in FIG. 1B, change in jitter amount with respect to the tilt intangential direction is steep about the point at which the tilt is 0.Accordingly, it is easy to find the optimum state in tilt adjustment,and, thus, to adjust the tilt optimally. However, as shown in FIG. 1A,change in jitter amount with respect to the tilt in radial direction isgentle about the point at which the tilt is 0. Accordingly, it isdifficult to find the optimum state in tilt adjustment, and, thus, toadjust the tilt optimally.

SUMMARY OF THE INVENTION

An object of the present invention is to provide a tilt detection systemby which it is possible to easily detect the optimum state in tiltadjustment also for radial direction similarly to the case fortangential direction, and to perform the tilt adjustment precisely andeasily.

Tilt detection according to the present invention comprises:

recording in an optical disk a series of data recorded, the series ofdata having a pulse width smaller than the minimum pulse width of datarecorded in an ordinary optical disk;

reproducing a signal from the optical disk by emitting a light beam ontothe optical disk; and

detecting a tilt of the light beam with respect to the optical diskbased on a reproduced RF signal provided by the reproducing part whenthe series of data having the smaller pulse width is reproduced.

In this configuration, as a result of a signal being reproduced from anoptical disk in which information inverts faster than data having theminimum pulse width recorded in an ordinary optical disk, the quality ofthe recorded data is degraded, and the quality of the reproduced signalsensitively responds to, change in the tilt. Accordingly, even in radialdirection, it is easy to detect the optimum state in tilt adjustmentsimilarly to the case of tangential direction, and thus to perform thetilt adjustment precisely and easily.

Tilt detection according to another aspect of the present inventioncomprises:

reproducing a signal from the optical disk;

decreasing a gain of a low-pass filter limiting a for the highestfrequency of predetermined essential frequencies of a reproduced RFsignal obtained from the reproduction the signal from the optical disk;and

detecting a tilt based on the signal having the above-mentioned highestfrequency having passed the low-pass filter.

In this configuration, the low-pass filter limits the frequency band ofthe signal obtained through reproducing, and decreases the amplitude ofthe signal of the above-mentioned highest frequency of the signalobtained through the reproduction. Thereby, the quality of the signal inof the highest frequency is degraded, and the quality of the reproducedsignal sensitively responds to change in the tilt. Accordingly, even inradial direction, it is easy to detect the optimum state in tiltadjustment similarly to the case of tangential direction, and to performthe tilt adjustment precisely and easily.

Tilt detection according to another aspect of the present inventioncomprises:

reproducing at a speed higher than an ordinary speed a signal from anoptical disk having data recorded therein in an ordinary recordingdensity; and

detecting the tilt based on a reproduced RF signal obtained through thereproduction.

In this configuration, data of the optical disk in which recording ismade at an ordinary speed is reproduced at a speed higher than theordinary speed. Thereby, the quality of the signal is degraded, and thequality of the reproduced signal sensitively responds to change in tilt.Accordingly, even in radial direction, it is easy to detect the optimumstate in tilt adjustment similarly to the case of tangential direction,and to perform the tilt adjustment precisely and easily.

Other objects and further features of the present invention will becomemore apparent from the following detailed description when read inconjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A and 1B show change in jitter amount with respect to tilt inradial direction and tangential direction in detection of tilt in therelated art;

FIG. 2 shows patterns recorded in an optical disk according to a firstembodiment of the present invention;

FIGS. 3A and 3B show the eye patterns obtained when data (FIG. 3A for 3Tpatterns and FIG. 3B for 2T patterns) recorded in the optical disk isreproduced;

FIG. 4 shows change in jitter amount with respect to tilt in radialdirection for illustrating the present invention;

FIG. 5 shows a circuit diagram of a tilt detecting circuit in the firstembodiment of the present invention;

FIG. 6 shows low-pass filters provided in an RF amplifier in each of asecond embodiment and a variant embodiment thereof of the presentinvention;

FIGS. 7A and 7B show gain characteristics of the low-pass filters shownin FIG. 6;

FIGS. 8A and 8B show circuit diagrams of tilt detecting circuits in thesecond embodiment and variant embodiment thereof of the presentinvention, respectively; and

FIGS. 9A and 9B show circuit diagrams of tilt detecting circuits in athird embodiment and a variant embodiment thereof of the presentinvention, respectively.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Embodiments of the present invention will now be described withreference to figures.

FIGS. 2 through 5 illustrate a first embodiment of the presentinvention.

In the first embodiment of the present invention, a successive 2Tpatterns, as shown in FIG. 2, are formed at a part of the track of anordinary CD-ROM disk for the tilt adjustment.

In the ordinary CD-ROM disk, an EFM-modulated signal is recorded in aform of pits. This signal has pulse widths of 3T through 11T where T isa duration corresponding to 1 channel bit. In contrast to this, in thetrack of the optical disk shown in FIG. 2, in addition to the patternscorresponding to the signal recorded in the ordinary optical disk asmentioned above, the patterns each having the length corresponding tothe pulse width 2T is formed which is narrower than the pulse width 3Twhich is the minimum one of those of the signal recorded in the ordinaryoptical disk.

FIG. 3A shows the eye pattern of the RF signal obtained when areproducing operation is performed on the ordinary optical disk, andFIG. 3B shows the eye pattern of the RF signal obtained when thereproducing operation is performed on the optical disk described withreference to FIG. 2.

For pulses of the signal having a narrow width, differential waves ofboth rise edge and decay edge thereof are interfered by one another,resolution is degraded, and the amplitude of waves obtained fromintegration thereof decreases. Accordingly, the amplitude B of the2T-pulse data shown in FIG. 3B of the narrow pulse width is smaller thanthe amplitude A of the 3T-pulse data shown in FIG. 3A.

Further, because the jitter amount increases as the level of thedetection signal decreases, the jitter amount of the reproduced signalof the 2T-pulse data is large.

In such a severe reproducing condition for the 2T-pulse data, thequality of reproduced signal responds to the tilt of the light beam withrespect to the recording surface of the optical disk (the term ‘of thelight beam with respect to the recording surface of the optical disk’thereof being omitted, hereinafter) sensitively.

FIG. 4 shows the jitter amount of the 2T-pulse data with respect to thetilt in radial direction.

As shown in the figure, change in jitter amount with respect to the tiltin radial direction is steep similar to change in jitter amount withrespect to the tilt in tangential direction shown in FIG. 1B.

Accordingly, it is possible to easily detect the optimum state in tiltadjustment also for radial direction, and to easily and preciselyperform the tilt adjustment.

FIG. 5 shows a circuit diagram of a tilt detection system in an opticaldisk drive in the first embodiment of to the present invention using theoptical disk described with reference to FIG. 2.

The reproduced light (reflected light) from the optical disk 1 rotatedby a disk motor 5 in a predetermined manner is detected by aphotodetector in an optical pickup 2, and is converted into an electricsignal therethrough, and, then, is input to an RF amplifier 3 whichamplifies the input signal in a predetermined manner. The thus-amplifiedsignal is input to a data pulse amplitude measurement device or jittermeter 4.

The data pulse amplitude measuring device detects the 2T-pulse data.Then the inclination of the pickup 2 or disk motor 5 is adjusted so thatthe thus-detected amplitude becomes maximum.

The inclination at which the amplitude is maximum is the optimuminclination (optimum state).

This adjustment of the inclination is performed for the two axes ofradial direction and tangential direction.

In the case of jitter meter, the 2T-data pulses are extracted from thebinary signal obtained from the reproduced RF signal, the number ofreference clock pulses occurring during each pulse width of theextracted 2T-data pulses is counted, and, thus, the jitter amount ismeasured.

Then, the inclination of the pickup 2 or disk motor 5 is adjusted sothat the thus-measured jitter amount becomes minimum.

The inclination at which the jitter amount becomes minimum is theoptimum inclination (optimum state).

This adjustment of inclination is performed for the two axes of radialdirection and tangential direction.

Thus, in the first embodiment, the data (2T-pulse data) having theinverting width shorter than that of the ordinary data is recorded inthe optical disk, and the thus-recorded data is reproduced.

By using the thus-obtained reproduced signal, it is possible to easilyand precisely detect the optimum state (inclination) of tilt adjustmentin radial direction.

However, because the 2T patterns are special patterns, it is necessaryto limit the frequency of occurrences of the patterns to the rangewithin which the position control system of the optical disk drive isnot adversely affected thereby.

A second embodiment and a variant embodiment thereof of the presentinvention will now be described with reference to FIGS. 6, 7A, 7B, 8Aand 8B.

In each of the second embodiment and variant embodiment, the tiltadjustment is performed in a condition in which the gain characteristicsof a low-pass filter of an optical disk drive are changed, and, thereby,the reproduction characteristics are made severe.

FIG. 6 shows low-pass filters included in an equalizer of an RFamplifier of the optical disk drive.

The low-pass filter 11 has the gain characteristics shown in FIG. 7Asuch that the gain becomes the peak about the frequency corresponding tothe above-mentioned 3T. In contrast to this, the low-pass filter 12 hasthe gain characteristics shown in FIG. 7B such that the gain decreasesabout the frequency corresponding to 3T.

As shown in FIG. 6, the low-pass filters 11 and 12 are connected inparallel, and one of them is selected by a change-over switch 13. Theswitch 13 is controlled by instructions provided by a CPU.

In an ordinary mode, the CPU provides the instructions to the switch 13such that the low-pass filter 11 is selected thereby. However, in atilt-adjustment mode, the CPU provides the instructions to the switch 13such that the low-pass filter 12 is selected thereby.

In the tilt-adjustment mode, the reproduced signal from the optical diskpasses through the low-pass filter 12, the frequency band of the signalis narrowed thereby, the amplitude of the 3T-pulse data decreases, theresolution is degraded, and both jitter amount and error rate increase.

As a result of the reproduction condition being thus made severe, theresponsivity of the quality of the reproduced signal to the tilt amountincreases. As a result, change in jitter amount with respect to the tiltin radial direction becomes steep so as to be similar to change injitter amount with respect to the tilt in tangential direction shown inFIG. 1B. Accordingly, it is possible to easily detect the optimum statein tilt adjustment in radial direction, and to easily and preciselyperform the appropriate tilt adjustment.

FIG. 8A shows a circuit configuration of a tilt detection system in theoptical disk drive of the above-described second embodiment.

A reproduced light (reflected light) from an optical disk 21 rotated ina predetermined manner by a disk motor 25 is detected by a photodetectorin an optical pickup 22, and is converted into an electric signaltherethrough, and, then, is input to the above-mentioned RF amplifier 23which amplifies the input signal in a predetermined manner. Thethus-amplified signal is input to a data pulse amplitude measurementdevice or jitter meter 24. Further, the CPU provides a control signal 26to the RF amplifier 23 for switching the low-pass filter includedtherein.

The low-pass filter 12 selected in accordance with the instructions fromthe CPU has the gain thereof about the frequency corresponding to 3Tlowered as mentioned above. Accordingly, the amplitude of the 3T-pulsedata output from the RF amplifier 23 is reduced. The data pulseamplitude measurement device detects the amplitude of the 3T-pulse data.

Then, the inclination of the pickup 22 or disk motor 25 is adjusted andthus the tilt adjustment is performed so that the thus-detectedamplitude becomes maximum. The inclination at which the amplitude ismaximum is the optimum inclination (optimum state).

This adjustment of the inclination is performed for the two axes ofradial direction and tangential direction.

In the case of jitter meter, the 3T-data pulses are extracted from thebinary signal obtained from the reproduced RF signal, the number ofreference clock pulses occurring during each pulse width of theextracted 3T-data pulses is counted, and, thus, the jitter amount ismeasured.

The inclination of the pickup 22 or disk motor 25 is adjusted and thusthe tilt adjustment is performed so that the thus-measured jitter amountbecomes minimum. The inclination at which the jitter amount is minimumis the optimum inclination (optimum state).

This adjustment of inclination is performed for the two axes of radialdirection and tangential direction.

FIG. 8B shows a circuit configuration of a tilt detection system in anoptical disk drive in the variant embodiment of the above-describedsecond embodiment of the present invention in which the tilt is detectedas a result of the error rate being detected.

A reflected light (reproduced light) from an optical disk 31 rotated ina predetermined manner by a disk motor 35 is detected by a pickup 32, isconverted into an electric signal therethrough, and, then, is amplifiedby an RF amplifier 33 (the same as the above-mentioned RF amplifier 23,and thus including the low-pass filters 11, 12 and changeover switch 13shown in FIG. 6) in a predetermined manner.

In the tilt adjustment mode, the low-pass filter 12 is selected inaccordance with the instructions from the CPU 36, and has the gainthereof about the frequency corresponding to 3T lowered as mentionedabove. Accordingly, the amplitude of the 3T-pulse data output from theRF amplifier 33 is reduced.

The thus-amplified signal is input to a signal processing circuit 34.The signal processing circuit 34 performs EMF demodulation of the inputsignal, and, then, performs error detection and correction using CIRC.The state of the error correction performed there, and thereby the errorrate, can be informed of to, and then be monitored through a CPU 36.

An operator thus monitors the error rate through the CPU 36 whileadjusting the inclination of the pickup 32 or disk motor 35 so that thethus-detected error rate becomes minimum, and, thus, performing the tiltadjustment.

Thus, the inclination of the pickup 32 or disk motor 35 is thus adjustedand thus the tilt adjustment is performed so that the thus-monitorederror rate becomes minimum. The inclination at which the error rate isminimum is the optimum inclination (optimum state).

This adjustment of inclination is performed for the two axes of radialdirection and tangential direction.

A third embodiment of the present invention will now be described.

In each of the third embodiment and a variant embodiment thereof, thetilt adjustment is performed in a condition in which an ordinary opticaldisk is rotated at a rotation speed higher than the ordinary rotationspeed, and, thereby, the reproduction characteristics are made severe.

FIG. 9A shows a circuit configuration of a tilt detection system in anoptical disk drive in the third embodiment of the present invention.

When the rotation speed of the optical disk is increased, the resolutionof the thus-obtained reproduced signal is degraded. At this time, the3T-data pulse having the narrow pulse width is affected therebyremarkably, the reproduced amplitude decreases, and both jitter amountand error rate increase.

As a result of the reproduction condition being thus made severe, changein amplitude/jitter amount with respect to the tilt becomes steep, and,thereby, the appropriate tilt adjustment can be performed easily andprecisely.

FIG. 9A shows the circuit configuration for the tilt detection performedas a result of the amplitude or jitter amount of the 3T-pulse data beingthus detected.

A reproduced light from the optical disk 41 rotated in a predeterminedmanner by a disk motor 45 is converted into an electric signal through aphotodetector in a pickup 42, and, then, is amplified by an RF amplifier43 in a predetermined manner. Thus thus-amplified signal is input to adata pulse amplitude measurement device or jitter meter 44.

The data pulse measurement device detects the amplitude of the 3T-pulsedata. The inclination of the pickup 42 or disk motor 45 is adjusted andthus the tilt adjustment is performed so that the thus-detectedamplitude becomes maximum. The inclination at which the amplitude ismaximum is the optimum inclination (optimum state).

FIG. 9B shows a circuit configuration of a tilt detection system in anoptical disk drive in the variant embodiment of the above-describedthird embodiment of the present invention in which the tilt is detectedas a result of the error rate being detected.

A reflected light (reproduced light) from an optical disk 51 rotated ina predetermined manner at the rotation speed higher than the ordinaryrotation speed by a disk motor 55 is detected by a pickup 52, isconverted into an electric signal therethrough, and, then, is amplifiedby an RF amplifier 53 in a predetermined manner.

The thus-amplified signal is input to a signal processing circuit 54.The signal processing circuit 54 performs EMF demodulation of the inputsignal, and, then, performs error detection and correction using CIRC.The state of the error correction performed there, and thereby the errorrate, can be informed of to, and then be monitored through a CPU 56.

An operator thus monitors the error rate through the CPU 56 whileadjusting the inclination of the pickup 52 or disk motor 55 so that thethus-detected error rate becomes minimum, and, thus, performing the tiltadjustment.

Thus, the inclination of the pickup 52 or disk motor 55 is thus adjustedand thus the tilt adjustment is performed so that the thus-monitorederror rate becomes minimum. The inclination at which the error rate isminimum is the optimum inclination (optimum state).

The present invention is not limited to the above-described embodiments,and variations and modifications may be made without departing from thescope of the present invention.

The present application is based, on Japanese priority application No.11-261588, filed on Sep. 16, 1999, the entire contents of which arehereby incorporated by reference.

What is claimed is:
 1. A tilt detection system comprising: a reproducingpart reproducing a signal from an optical disk by emitting a light beamonto said optical disk; a low-pass filter having a gain decreased forthe highest frequency of predetermined essential frequencies of areproduced RF signal provided by said reproducing part; and a tiltdetecting part detecting a tilt of the light beam with respect to theoptical disk based on the signal having said highest frequency havingpassed said low-pass filter.
 2. An optical disk drive comprising: areproducing part reproducing a signal from an optical disk by emitting alight beam onto said optical disk; a low-pass filter having a gaindecreased for the highest frequency of predetermined essentialfrequencies of a reproduced RF signal provided by said reproducing part;and a tilt detecting part detecting a tilt of the light beam withrespect to the optical disk based on the signal having said highestfrequency having passed said low-pass filter.
 3. A tilt detection methodcomprising the steps of: a) reproducing a signal from an optical disk byemitting a light beam onto said optical disk through a reproducing partof an optical disk drive; b) decreasing a gain of a low-pass filter ofsaid optical disk drive having for the highest frequency ofpredetermined essential frequencies of a reproduced RF signal providedby said reproducing part; and c) detecting a tilt of the light beam withrespect to the optical disk in said reproducing part based on the signalhaving said highest frequency having passed said low-pass filter.
 4. Atilt adjustment method comprising the steps of: a) reproducing a signalfrom an optical disk by emitting a light beam onto said optical diskthrough a reproducing part of an optical disk drive; b) decreasing again of a low-pass filter of said optical disk drive having for thehighest frequency of predetermined essential frequencies of a reproducedRF signal provided by said reproducing part; c) detecting a tilt of thelight beam with respect to the optical disk in said reproducing partbased on the signal having said highest frequency having passed saidlow-pass filter; and d) adjusting a mechanical configuration of saidoptical disk so that the tilt detected in said step c) is minimized.